Generation of electricity and the treatment and disposal of sewage

ABSTRACT

The invention provides a method and apparatus for the generation of electricity combined with the further treatment of sewage from a sewage treatment plant. In one form of the invention a stream of compressed air/contaminated air ( 108, 110 ) is passed to a mixing device ( 18 ) where it is mixed with a stream of introduced fuel gas ( 112 ). The mixture is combusted and the resultant gases are fed to the expansion chamber ( 6 ) of a gas turbine which drives an alternator ( 10 ) thus generating electricity. Rather than simply dissipate them to atmosphere, the gases ( 126 ) are utilized in the heating and disinfection of sewage ( 148 ) from an initial sewage treatment plant. The thus disinfected sewage ( 150 ) passes to a pressure vessel ( 12 ) wherein it encounters and saturates the stream of compressed air/contaminated air ( 108, 110 ) passing to the mixing device ( 18 ).

BACKGROUND

The average consumption of power in a typical Australian community isabout 1 kWhr per hour per person. The production of sewage is about 200litres per day per person. At present, the provision of electricity isnot seen as being complementary to, or logically associated with, thetreatment and possible re-use of sewage.

Sewage

Treated sewage, in many instances, is simply discharged into the sea orinto inland river systems and the enhanced nitrogen and phosphoruscontent of sewage, as opposed to the raw water supplied to households,is associated with the production of algal “blooms” in rivers andwaterways. This has resulted in attention being focussed on reducing thenitrogen and phosphorus content of treated sewage. Reductions are ofcourse possible with complex sewage treatment and/or includingprocessing operations such as the addition of chemicals and furthersettling, biological processes, filtration operations and the like.

It is possible to use treated sewage with a high nitrogen and phosphoruscontent for irrigation, and in the production of crops, fodder and thegrowth of trees, lawns, gardens and the like, effective use being madeof the nitrogen and phosphorus content and the water run off havingreduced levels of nitrogen and phosphorus. However a problem with theuse of treated sewage for irrigation is that the colliform and pathogencontent can typically be about 100-200 units per 100 ml. This can resultin a number of problems and disadvantages (such as a significant healthrisk, the relocation of people during and for a period after theapplication of treated sewage, the non use of resultant agriculturalproduce for human consumption unless the end product is cooked undercontrolled conditions). For these reasons treated sewage cannot begenerally used for normal agriculture, accessible parklands, externalwashing purposes or as a household “grey” water.

Further, in some locations in NSW for example, whilst there is a demandfor approval of building lots for further construction of houses, suchapprovals are being impeded by the effluent problems associated withconventional sewage treatment plants. There is also the perceived needto preserve and enhance the regions near inland creeks and waterways soas to provide visually attractive “green belts” and recreational areaswhich, under normal conditions, require the use of substantial amountsof water for strong plant and tree growth.

It is known that the term sewage may also incorporate various industrialeffluents, food processing wastes, agricultural wastes and other wastewater streams which may or may not contain pathogens and the like butwhich may be mixed with household type sewage. Again, this cannot beused with safety.

It is also known that the anaerobic digestion of sewage converts sewageorganic matter to methane and carbon dioxide. However, due to its highcarbon dioxide content and low pressure this gas can be expensive tocompress to enable its use in gas turbines. Anaerobic digestion requiresheat which can be costly.

A still further problem with sewage plants is the associated gas andodour releases. It is known that sewage sludge produced in sewagetreatment decomposes to produce methane and foul smellingsulphur-containing gases such as hydrogen sulphide. There is a need toeliminate the foul odours and if possible utilize these gases.

Electricity

A further general problem with population growth and new housing is therecognised need to reduce greenhouse gas emissions, which arepredominantly associated with the production of electricity. In recentyears the trend to install large coal-fired power stations remote frompopulous areas has been the preferred mode of supplying power (e.g., inNSW). However, this system has the problem of creating substantialgreenhouse gas emissions compared to gas-fuelled power plant and theneed for high tension transmission corridors which are now seen asunsightly and a potential source of electromagnetic radiation associatedwith unquantifiable health risks. A further problem with coalfield-located coal-fired power plant, and long high voltage transmissionsystems, is the appreciable amount of power lost in electricitytransmission.

Whilst changing from remote coal-fired power plant to gas-fired planthas the desired benefit of reducing carbon dioxide emissions associatedwith power generation, the generation of gas-based power in populousareas has the disadvantage of discharging nitric oxide (NOX) emissionsfrom power generation in such areas (it is known that NOX emissions area precursor to a range of deleterious tropospheric reactions such asozone and smog formation). In many populous areas strict controls areplaced on power generation plant with a best achievable NOX content ofthe exhaust from gas turbines being, typically, about 20 ppm. This maybe regarded as being acceptable by some authorities. However there is aperceived need, and a known problem, in lowering NOX emissions and amajor problem in being able to lower emissions in gas turbine exhaustsbelow 5 ppm. For example, Los Angeles, which is known to have severetropospheric pollution problems, has a current requirement for NOXdischarges in gas turbines to be less than 5 ppm. At present this levelof NOX emissions cannot be achieved with commercially available gasturbines.

The Invention

By means of this invention the described problems associated with thedisposal of sewage and treated sewage, and the generation of power areaddressed and, at least in part, are solved.

Accordingly, in one aspect, the invention provides a method for thegeneration of electricity and the complementary purification of sewagecharacterized in that:

(I) exhaust gases, from a mixture of compressed air or compressed airand contaminated air and fuel gas combusted in a gas turbine for thegeneration of electricity, are utilized in a first step of heating anddisinfecting a stream of sewage from a sewage treatment plant; and

(ii) the thus disinfected hot sewage stream is contacted with, andsaturates, a stream of the compressed air or compressed air andcontaminated air prior to its admixture with the fuel gas.

In a further aspect, the invention provides a method for the productionof electricity and the complementary purification of sewage comprisingthe following steps:

(I) introducing, into a heating vessel, a stream of sewage from a sewagetreatment plant;

(ii) introducing, into the compressor of a gas turbine, a stream of airor optionally a stream of air and contaminated air;

(iii) conveying the compressed air or compressed air and contaminatedair to a mixing device wherein the compressed air/contaminated air ismixed with a stream of fuel gas separately introduced at the mixingdevice;

(iv) combusting the mixture of compressed air/contaminated air and fuelgas to produce hot gases which are fed to the expansion stage of theturbine which drives an alternator to produce electricity;

(v) conveying exhaust gases from the expansion stage of the turbine tothe heating vessel containing the introduced stream of sewage, thesewage being heated to a temperature, and retained at the temperaturefor a period of time, sufficient to disinfect the sewage and thus ensurethat all pathogens and bacteria in the sewage are effectively destroyed;

(vi) introducing the thus disinfected hot sewage to a pressure vesselwhere it is brought into contact with, and saturates, the stream ofcompressed air or compressed air/contaminated air being conveyed fromthe compressor to the mixing device; and

(vii) discharging the thus purified and disinfected sewage for use asrequired.

There is also provided, in accordance with the invention, apparatus forthe generation of electricity and the complementary purification ofsewage comprising in combination:

(I) a heating vessel;

(ii) means for introducing a stream of treated sewage to the heatingvessel;

(iii) means for heating the treated sewage in the heating vessel todisinfect the treated sewage, the heating means comprising the exhaustgases from a mixture of separately introduced compressed air orcompressed air and contaminated air and fuel gas combusted in a gasturbine to generate electricity;

(iv) means for conveying the introduced pre-mixed compressedair/contaminated air to a device where it is mixed with the introducedfuel gas;

(v) means for introducing the hot disinfected sewage into the compressedair/contaminated air conveying means, the disinfected sewage therebycoming into contact with and saturating the compressed air/contaminatedair being conveyed to the mixing device.

The invention further provides apparatus for the generation ofelectricity and the complementary purification of sewage comprising incombination:

(I) a heating vessel;

(ii) means for introducing a stream of treated sewage to the heatingvessel;

(iii) a gas turbine comprising a compression stage, for compression ofair or air and contaminated air, and an expansion stage which drives thecompression stage and an alternator to generate electricity;

(iv) means for conveying the compressed air or compressed air andcontaminated air through a pressure vessel, wherein it is saturated, toa mixing device where the compressed air/contaminated air is mixed witha separately introduced stream of fuel gas;

(v) a combustion chamber wherein the mixture of compressedair/contaminated air and fuel gas is combusted and from whence the hotresultant gases are conveyed to the expansion stage of the gas turbine;

(vi) means for heating the treated sewage in the heating vessel todisinfect the heated sewage, the heating means comprises the exhaustgases from the expansion stage of the gas turbine;

(vii) means for introducing the hot disinfected sewage into thecompressed air/contaminated air conveying means, the disinfected sewagethereby coming into contact with and saturating the compressed air/contaminated air being conveyed to the mixing device.

The invention will now be described with sequential reference to (I)preferred sub-generic features, (ii) a specified embodiment itselfdescribed with reference to the accompanying schematic drawing, and(iii) a working example (likewise described in conjunction with andreference to the illustrated drawing). The various integer(s)constituting the preferred embodiments of the several means referred toabove will be readily identifiable in the detailed description anddrawing. It is to be understood that, being in respect ofpreferred/illustrative features, this description should not belimitatively construed.

For convenience, the expression “compressed air or compressed air andcontaminated air” may frequently be simply expressed as “compressedair/contaminated air”.

In preferred forms of the invention:

The temperature to which the sewage is heated—to ensure disinfection ofthe sewage and destruction of resident pathogens, colliforms and thelike—is generally in the range 100-180° C., more preferably 130-150° C.The residence time at the operating temperature is typically in excessof five minutes.

The turbine is a recuperated gas turbine.

The mixture of compressed air/contaminated air and fuel gas is combustedin a combusted of the type described in co-pending applicationPCT/AU95/00719.

The contaminated air introduced into the compressor of the turbine iscomprised of foul air and methane gas from the sewage treatment plant(the foul air/methane being thereby incinerated with distribution of thefoul odours and utilization of the energy value of the methane).

The hot disinfected sewage is introduced into the pressure vessel inwhich it comes into contact with the compressed air/contaminated airpassing through the vessel to the mixing device, by means of a spraysystem/liquid distribution device.

The pressure vessel is also equipped with gas/liquid contacting misteliminator systems through which the saturated stream ofcompressed/contaminated air passes en route to the mixing device.

a portion of the stream of treated sewage passing from the sewagetreatment plant to the heating vessel is diverted to a cooling coil inthe pressure vessel and thence back to the stream.

The fuel gas may be derived from coal or other solid carbonaceous fuelgasification as described in co-pending application PCT/AU96/00483.

Proceeding to the description of the particular embodiment which isschematically illustrated in accompanying FIG. 1 of the drawings, theapparatus by means of which the method of the invention is carried out,and the components thereof, are first identified by reference numerals(and, as appropriate, brief indications of their respective functions).Thereafter the preferred method of generation/production isoperationally described.

In FIG. 1 item-2 is the compression stage of a gas turbine, numeral 4denotes the shaft connecting compression stage 2 to the expansion stage6 of the gas turbine which drives both the compressor 2 and, via shaft8, an alternator 10. The alternator 10 generates electricity. A pressurevessel 12 houses a spray system or liquid distribution device 28, andalso two gas/liquid contacting mist eliminator systems 14 and 16. Thesesystems are of a type well known in the art and may be of proprietaryknitted mesh or constituted by mist eliminator pads. Their function willbe explained below. Item-18 denotes a mixing device for air and fuelgas, such as natural gas, 20 denotes a recuperator and 22 denotes acombustion chamber. In this embodiment of the invention, the combustionchamber is of the type described in co-pending patent application Ser.No. PCT/AU95/00719. The chamber is fed with a pre-mixed air and fuel gasmixture.

Item-24 denotes an exhaust gas-fed vessel for heating incoming sewage.Its function will be elaborated below. Item-26 denotes a sewage storagetank likewise to be further elaborated.

Air is fed to the gas turbine compressor 2 via duct 102.Odour-contaminated, and possibly methane-containing air from a sewagetreatment plant, is also added via duct or pipeline 104. The thus mixedair and contaminated air is fed to the gas turbine compressor 2 via duct106. Duct 106 may contain a filtration device or devices.

Compressed air and compressed contaminated air leaves the gas turbinecompressor 2 via pipeline 108 and passes to the pressure vessel 12 wherethe upwardly moving air is contacted with hot disinfected sewage fromthe spray system/liquid distribution device (see further below). Thethus saturated and adiabatically cooled air then passes through the misteliminator 14, and then passes over cooling coils/pipeline 144.Condensate from the coil 144 drops down and through the mist eliminator14, thereby washing and assisting in the substantial removal of saltscontaining liquid droplets and any solids introduced through the liquidspray/distribution device 28. The now saturated air then passes throughmist eliminator 16 to remove any remaining water droplets before passingvia pipeline 110 to gas mixing device 18.

Fuel gas for the turbine, to be combined with the gas travelling viapipeline 110, is fed via pipeline 112 and control valve 114 and pipeline116 to the fuel gas and air mixing device 18. The control of the gasflow by valve 114, is carried out in known manner. The fuel gas and airmixture which is substantially below the lower explosive limit for themixture is fed to the combustion chamber 22 via recuperator 20 where itis preheated. Recuperation prior to combustion increases the efficiencyof the turbine. In the combustion chamber the fuel gas is combusted andthe hot, resultant gases are fed via duct 122 to the expansion stage 6of the turbine. The expansion stage drives the compressor 2, and thealternator 10, which generates electricity.

Exhaust gases from the expansion stage 6 leave via duct 124 and pass tothe recuperator 20 where they are partially cooled. The partially cooledexhaust gases then pass via duct 126 to the heater 24 where they performthe heating function to be further explained. On leaving the heater theyare exhausted to atmosphere via duct 128.

Treated sewage from a sewage treatment plant, which may typicallycontain in excess of 100 colliform units per 100 ml is introduced underpressure via pipeline 130 to a control valve 132 which controls the rateof flow. The sewage then passes via pipeline 134 and restriction deviceor restriction orifice 136, and thence via pipeline 148 to the heater24. Part of the treated sewage in pipeline 134 is diverted via pipeline138 via control valve 140 and pipeline 142 to the cooling coil 144 andthence via pipeline 146 back to pipeline 148. The flow of the sewage incooling coil 144 is sufficient to cool the saturated air leaving misteliminator 14 to below its saturation temperature, prior to passing overthe cooling coil so as to produce a flow of substantially purecondensate to reflux over the surface of mist eliminator 14 and wash anydissolved solids and entrained solid particles from the ascendingsaturated air.

The treated sewage entering the heater 24 is heated to 130 to 150° C.and then passes via pipeline 150 to the storage tank 26. The temperaturein the heater 24 and the storage tank 26 is maintained at 130-150° C.The residence time at this high temperature ensures that all pathogensand bacteria in the treated sewage are effectively destroyed by knownautoclaving techniques.

In operation, the storage tank 26 will normally be full of pressurised,heated liquid sewage. However, in the event of some malfunction (e.g.,poor or faulty control causing the sewage to overheat and start to boil)phase separation will occur in tank 26 and a level device (not shown)will detect the presence of steam. System control mechanisms (not shown)can then, for example, increase the sewage input flow through controlvalve 132 and/or open safety valve 154 to allow steam to escape viapipelines 152 and 156 (to atmosphere at a suitable and safe location).

In an alternative (not illustrated) hot turbine exhaust gas can beby-passed around the heater 24, emergency level controls and alarms willshut off the various valves and other mechanisms. Such safety devices(to protect the overall system and to comply with safety regulations)will be within the knowledge of those skilled in the art of processdesign and plant safety.

The hot, disinfected sewage leaving storage tank 26 passes via pipeline158, control valve 160 and pipeline 162 to the spray system/liquiddistributer 28 in pressure vessel 12. Here, the hot treated anddisinfected sewage passing downwardly, is contacted with and saturatesthe compressed air passing upwardly in vessel 12. For the sake of simpleillustration, the sewage air contact zone in vessel 12 is simply shownas an open space. However, it will be understood that a known contactingsystem (such as distillation-type trays, a packing arrangement such asrashig rings or the like, a disk and doughnut-type tray arrangement orother known liquid/gas contacting device or devices) is employed.

The control valve 160, controlling the passage of the treateddisinfected sewage, can be a pressure control device. It will maintain asuitable pressure and temperature of the sewage, leaving heater/tank24/26, in a single, liquid phase at up to typically 150° C. (and belowthe boiling point of the liquid at that pressure).

The hot, disinfected, treated sewage leaves the pressure vessel 12 viapipeline 164, control valve 166, and pipeline 168. It passes to a sewagedisposal system (now shown) which can typically be a cooler and storagetank. This hot treated sewage can be used as a source of process orbuilding heating. It may also be used to pre-heat incoming sewage.

Maintenancewise, the pipes in the heater 24 may be periodically cleanedby the injection of ass beads or sand or the like into pipeline 130 orpipeline 148. The sand or beads can be separated in vessel 26 which maybe suitably modified for the purpose. Beads or sand moved from vessel 26may be recycled for re-use.

In Operation

Sewage, from a sewage treatment plant, is introduced via pipe 130 andpasses to a hot water heater 24. In the heater it is subjected toexhaust gases (see below). The sewage from heater 24 travels via pipe150 to heated sewage storage tank 26.

The sewage in heater 24 and in storage tank 26 is maintained at elevatedtemperature (e.g., 130-150° C.) for a sufficient time to ensure that allpathogens and bacteria in the sewage are destroyed.

The disinfected sewage then passes from the tank 26 via pipeline 158/162to a spray distribution device 28 in the pressure vessel 12. The sewageexits device 28 and passes downwardly through, and saturates, a streamof upwardly rising compressed air (see below) from compressor 2. Thesewage exits the apparatus from pipe/valve 164/168.

The exhaust gases that pass through the water heater are derived asfollows: Ordinary air mixed with contaminated air enters the compressor2 of a turbine. The compressed air/contaminated air passes upwardlythrough pressure vessel 12 (where it encounters the sewage exiting thespray decompression device 28) and thence, as saturated air via pipe110, to a mixing device 18 where it is mixed with fuel gas for theturbine introduced at pipeline 112. The saturated air/fuel gas mixturepasses to combustion chamber 22. The added water vapour increases themass flow passing to the combustion chamber. The hot resultant gasesfrom the combustion chamber pass to the expansion stage 6 of the turbinewhich drives both the compressor 2 and an alternator 10 which generateselectricity. The exhaust gases pass to the heater 24, performing thefunction as above described, and thence to atmosphere.

Summarising the above operation, the generation of electricity iscombined with the further treatment of sewage from a sewage treatmentplant. A stream of compressed air/contaminated air is passed to a mixingdevice where it is mixed with a stream of introduced fuel gas. Themixture is combusted and the resultant gases are fed to the expansionchamber of a gas turbine which drives an alternator thus generatingelectricity. Rather than simply dissipate them to atmosphere, theexhaust gases are utilized in the heating and disinfection of sewagefrom an initial sewage treatment plant. The thus disinfected sewagepasses to a pressure vessel wherein it encounters and saturates thestream of compressed air/contaminated air passing to the mixing device.

In the foreshadowed example of the invention which is presented withreference to accompanying FIG. 1:

(a) The gas turbine system, items 2, 4, 6, 8, 10, 20, is a recuperated3000R Centaur turbine (as manufactured by Solar Gas Turbines).

(b) The combustor 22 is as described in co-pending patent applicationSer. No. PCT/AU9500719.

(c) Item-12 is a pressure vessel of 1.2 metres diameter with the lowerzone below.

(d) Item-14 is 3 metres high above the dished end with the lower 0.5metres being the separated and partially cooled sewage. It is 0.5 metresof “knitted” metal mesh.

(e) Item-16 is a further 0.5 metres of “knitted” metal mesh designed toeliminate mist particles down to and including particles of 1 micron.

(f) Item-144 is 75 metres of 25 mm n.b. pipe arranged in 5 horizontal“pancakes” of pipe spaced at 80 mm centres with a flowrate of 0.6 litresper second of sewage inside the pipe.

(g) Item-26 is a vertical cylindrical vessel of 1.5 metres dia and 3.0metres between dished ends.

(h) Item-24 is a rectangular shaped heater of mild steel constructionwith internal insulation and internal, overlapping, expandable sheetsteel lining with 5 horizontal gas passes each 10.5 metres long and1.2×1.2 metre internal cross section.

(i) The sewage heating pipe 148 is a single pass of 80 mm n.b. (88.9 mmo.d.) steel pipe on 137 mm square pitch arranged in 5 bundles, onebundle per gas pass with each bundle having 64 tubes with U-bends andeach tube 10 metres long, i.e., a total straight tube length of 3,200metres.

Air temperature leaving the compressor, Item-2 288° C. Air/water vapourtemperature leaving item-16 120° C. Exhaust temperature leaving item-24150° C. Temperature in pipeline 150 140° C. Temperature exit item-22,inlet item-6 871° C. The ISO rating of a standard unmodified turbine2.55 MW The ISO efficiency of a standard unmodified turbine 32% Air flowto the turbine 17.9 Kg per second The untreated sewage flow passing viapipeline 130 8.5 liters per second The ISO efficiency of the modifiedturbine 39% The ISO power rating of the modified turbine 3.68 MW Watervapour added to air flow leaving item-12  1.5 Kg per second Hot, treatedsewage leaving pipeline 168 7.0 liters per second Pressure upstream ofvalve 160 700 kPa absolute NOX in exhaust leaving 128 less than 5 p.p.m.

It will of course be understood that, within the ambit of the invention,the above described particular embodiment can be varied and extended inmany respects. For example:

Selected bacteria can be added to the disinfected sewage, which can beheld under optimum conditions of temperature and aeration for rapidgrowth of said bacteria, to aid decomposition of soluble and particulatematter in the effluent.

To further increase the evaporation of water vapour from the sewage, andthe power output of the gas turbine, the partially cooled sewage can bereturned, further heated by turbine exhaust gases, and reinjected intothe saturated compressed air.

The hot disinfected sewage leaving the apparatus can be used directly orindirectly to provide industrial process, commercial or householdheating.

At least part of the heat in the hot disinfected sewage leaving theapparatus can be used to heat and aid anaerobic digestion of sewagewithin the sewage plant.

In conclusion the invention provides an improvement over the current,commercial means of providing power, namely conventional coal or gasturbine power stations. In addition, treated sewage can be safelyre-used for gardening, exterior washing, toilet flushing and the likerather than simply discharged to waste. It will thus be seen that, bythe invention, a substantial contribution to the art has been made.

The claims defining the invention are as follows:
 1. A method for thegeneration of electricity and the complementary purification of sewage,comprising: (i) heating and disinfecting a stream of sewage from asewage treatment plant utilizing exhaust gases from a mixture of air andfuel gas combusted in a gas turbine for the generation of electricity;and (ii) contacting the thus disinfected hot sewage stream with, andsaturating, a stream of the air prior to its admixture with the fuelgas.
 2. A method for the production of electricity and the complementarypurification of sewage comprising the following steps: (i) introducing,into a heating vessel, a stream of sewage from a sewage treatment plant;(ii) introducing, into the compressor of a gas turbine, a stream of air;(iii) conveying the compressed air to a mixing device wherein thecompressed air is mixed with a stream of fuel gas separately introducedat the mixing device; (iv) combusting the mixture of air and fuel gas toproduce hot gases which are fed to the expansion stage of the turbinewhich drives an alternator to produce electricity; (v) conveying exhaustgases from the expansion stage of the turbine to the heating vesselcontaining the introduced stream of sewage, the sewage being heated to atemperature, and retained at the temperature for a period of time,sufficient to disinfect the sewage and thus ensure that all pathogensand bacteria in the sewage are effectively destroyed; (vi) introducingthe thus disinfected hot sewage to a pressure vessel where it is broughtinto contact with, and saturates, the stream of air being conveyed fromthe compressor to the mixing device; and (vii) discharging the thuspurified and disinfected sewage for use as required.
 3. A method asclaimed in claim 2 wherein the disinfected hot sewage is introduced tothe pressure vessel through a spray liquid distribution device.
 4. Amethod as claimed in claim 2 wherein, following saturation, a stream ofsaturated air traverses a liquid contacting mist eliminator system inthe pressure vessel whereby mist, liquid droplets and any solidparticles introduced through the liquid spray distribution device areremoved.
 5. A method as claimed in claim 2 wherein a portion of thestream of treated sewage passing to the heating vessel is diverted to acooling coil above a mist eliminator system in the pressure vessel andthence back to the stream and the cooled gases pass through a furthermist eliminator system above the cooling coil such that condensed waterdroplets are recovered and flow down through the initial misteliminator.
 6. A method according to claim 1 further comprising coolingthe air stream using a cooling medium.
 7. A method as claimed in claim 2wherein the gas turbine is a recuperated gas turbine.
 8. A method asclaimed in claim 2 wherein, in the step of heating and disinfecting thetreated sewage, the sewage is heated to a temperature of 100-180° C.,and retained at that temperature for a period in excess of five minutes.9. A method as claimed in claim 8 wherein the temperature range is130-150° C.
 10. Apparatus for the generation of electricity and thecomplementary purification of sewage comprising in combination: (i) aheating vessel; (ii) means for introducing a stream of treated sewage tothe heating vessel; (iii) means for heating the treated sewage in theheating vessel to disinfect the treated sewage, the heating meanscomprising the exhaust gases from a mixture of separately introduced airand fuel gas combusted in a gas turbine to generate electricity; (iv)means for conveying the introduced pre-mixed air to a device where it ismixed with the introduced fuel gas; (v) means for introducing the hotdisinfected sewage into the air conveying means, the disinfected sewagethereby coming into contact with and saturating the air being conveyedto the mixing device.
 11. Apparatus for the generation of electricityand the complementary purification of sewage comprising in combination:(i) a heating vessel; (ii) means for introducing a stream of treatedsewage to the heating vessel; (iii) a gas turbine comprising acompression stage, for compression of air, and an expansion stage whichdrives the compression stage and an alternator to generate electricity;(iv) means for conveying the compressed air through a pressure vessel,wherein it is saturated, to a mixing device where the compressed air ismixed with a separately introduced stream of fuel gas; (v) a combustionchamber wherein the mixture of compressed air and fuel gas is combustedand from whence the hot resultant gases are conveyed to the expansionstage of the gas turbine; (vi) means for heating the treated sewage inthe heating vessel to disinfect the heated sewage, the heating meanscomprises the exhaust gases from the expansion stage of the gas turbine;(vii) means for introducing the hot disinfected sewage into thecompressed air conveying means, the disinfected sewage thereby cominginto contact with and saturating the compressed air being conveyed tothe mixing device.
 12. Apparatus as claimed in claim 11 wherein thepressure vessel incorporates a spray liquid distribution device throughwhich the hot disinfected sewage is fed into contact with the compressedair.
 13. Apparatus as claimed in claim 12 which comprises a storage tankin which the hot disinfected sewage is held and through which it isconveyed to the spray liquid distribution device.
 14. Apparatus asclaimed in claim 11 wherein the pressure vessel incorporates a liquidcontacting mist eliminator system through which the saturated compressedair passes.
 15. Apparatus as claimed in claim 11 further comprisingmeans for diverting a portion of the stream of heated sewage passingfrom the sewage treatment plant to the heating vessel, to a cooling coilin the pressure vessel and thence back to the stream.
 16. Apparatus asclaimed in claim 10 wherein the gas turbine is a recuperated gasturbine.
 17. The method as claimed in claim 1, wherein the air comprisesat least one of fresh air and contaminated air.
 18. The method asclaimed in claim 2, wherein the air comprises at least one of fresh airand contaminated air.
 19. The apparatus as claimed in claim 10, whereinthe air comprises at least one of fresh air and contaminated air. 20.The apparatus as claimed in claim 11, wherein the air comprises at leastone of fresh air and contaminated air.
 21. The method as claimed inclaim 1, wherein in the step of heating and disinfecting the sewage ismaintained at a temperature below the boiling point of the sewage atambient pressure to inhibit gas evolution.